CN108630910B

CN108630910B - Electrode material for lithium ion secondary battery and lithium ion secondary battery

Info

Publication number: CN108630910B
Application number: CN201710874731.2A
Authority: CN
Inventors: 休石纮史; 山屋竜太
Original assignee: Sumitomo Osaka Cement Co Ltd
Current assignee: Sumitomo Metal Mining Co Ltd
Priority date: 2017-03-24
Filing date: 2017-09-25
Publication date: 2020-03-06
Anticipated expiration: 2037-09-25
Also published as: EP3379615A1; JP6288340B1; US20180277887A1; CN108630910A; JP2018163763A; EP3379615B1

Abstract

The invention provides an electrode material for a lithium ion secondary battery, which can reduce the volume resistance value of an electrode mixture layer and the interface resistance value of the electrode mixture layer and an aluminum current collector, and a lithium ion secondary battery with improved charge and discharge characteristics. The electrode material for a lithium ion secondary battery of the present invention comprises: an electrode active material including an olivine-structured transition metal lithium phosphate compound; and a carbonaceous coating film covering the electrode active material, the electrode active material having a specific surface area of 10m²More than 25 m/g²(ii) not more than g, the content of spherical secondary particles having an average particle diameter of 0.5 to 15 [ mu ] m and a circularity of 0.90 to 0.95, which are obtained by granulating the primary particles of the electrode active material, is not less than 18% by number ratio relative to the total number of particles including a single particle and all the spherical secondary particles when the circularity is measured. The lithium ion secondary battery of the present invention has a positive electrode mixture layer in which the electrode material of the present invention is used for the positive electrode.

Description

Electrode material for lithium ion secondary battery and lithium ion secondary battery

Technical Field

The present invention relates to an electrode material for a lithium ion secondary battery and a lithium ion secondary battery including an electrode formed using the electrode material.

Background

Lithium ion secondary batteries have higher energy density and output density than lead batteries and nickel metal hydride batteries, and are used in various applications such as household backup power supplies and electric tools, as represented by small electronic devices such as smartphones. In addition, the lithium ion secondary battery has been put to practical use as a large-capacity lithium ion secondary battery for storing renewable energy such as solar power generation and wind power generation.

A lithium ion secondary battery includes a positive electrode, a negative electrode, an electrolyte solution, and a separator. As an electrode material constituting the positive electrode, lithium cobaltate (LiCoO) can be used₂) Or lithium manganate (LiMn)₂O₄) Lithium iron phosphate (LiFePO)₄) Etc. having a property of reversibly deintercalating and intercalating lithium ions, and have a high capacity and a long life from a batteryImprovements have been studied from various viewpoints such as improvement of safety and cost reduction.

Lithium iron phosphate (LiFePO) of the electrode material₄) Since iron, which is abundant in resources and inexpensive, is used, it is a material that is easily reduced in cost. Lithium iron phosphate has excellent characteristics not possessed by oxide-based positive electrode materials represented by lithium cobaltate, for example, because it does not emit oxygen gas at high temperatures due to a strong covalent bond between phosphorus and oxygen, and therefore has remarkable safety.

On the other hand, lithium iron phosphate has lower Li ion diffusibility and electron conductivity, and thus has inferior input/output characteristics compared to oxide-based positive electrode materials. This characteristic difference is more pronounced when the operating temperature of the battery is low, and therefore lithium iron phosphate is considered to be unsuitable for use in vehicle-mounted applications such as hybrid vehicles that require high input-output characteristics in a low temperature region.

LiMPO having olivine structure represented by lithium iron phosphate₄Since the Li ion (M is a metal element) has low diffusion property and electron conductivity, it can pass through LiMPO₄The primary particles are made finer, and the surfaces of the primary particles are coated with a conductive carbonaceous coating film, thereby improving the charge and discharge characteristics.

On the other hand, the above-mentioned micronized LiMPO₄Because of the large specific surface area of (a), thickening of the electrode mix slurry and a large amount of binder are required, and for this reason, the properties of the electrode mix slurry are generally improved by granulating primary particles coated with a carbonaceous coating as secondary particles.

For example, patent document 1 discloses, as an electrode material, a positive electrode active material for a lithium secondary battery, which has a matrix particle comprising a lithium nickel manganese-based composite oxide in which primary particle crystals aggregate to form a spherical secondary particle and which has a void on the surface and inside of the secondary particle, and a conductive fine powder filling a part of the void of the matrix particle. Patent document 2 discloses a positive electrode active material for a lithium secondary battery containing particles having voids in the interior of secondary particles.

Patent document 1: japanese laid-open patent publication No. 5343347

Patent document 2: japanese laid-open patent publication (Kokai) No. 2015-018678

Disclosure of Invention

Problems to be solved by the invention

The electrode mixture layer is formed by applying an electrode slurry in which an electrode material, a conductive assistant, a binder, and the like are mixed to an aluminum current collector through the current collector, drying, and pressing. However, in the electrode materials described in patent documents 1 and 2, since the secondary particles are amorphous or voids (hollow secondary particles) are present in the secondary particles, the electrode structure becomes uneven, and Li ion conductivity and electron conductivity are lowered. Further, excessive pressing is required to make the electrode structure uniform, and peeling of the conductive carbonaceous coating due to collapse of the secondary particles, or deterioration of battery characteristics due to peeling of the electrode mixture layer from the aluminum current collector, or the like, may also occur.

In this way, in order to improve charge/discharge characteristics, it is necessary to improve not only the electrode material but also the conductivity of the electrode material mixture layer constituting the electrode.

The present invention has been made in view of the above circumstances, and an object thereof is to provide an electrode material for a lithium ion secondary battery capable of reducing the volume resistance value of an electrode mixture layer and the interface resistance value between the electrode mixture layer and an aluminum current collector, and a lithium ion secondary battery having improved charge and discharge characteristics.

Means for solving the problems

As a result of intensive studies to solve the above problems, the present inventors have found that the problems can be solved by the following invention.

[1]An electrode material for a lithium ion secondary battery, comprising: an electrode active material including an olivine-structured transition metal lithium phosphate compound; and a carbonaceous coating film covering the electrode active material, wherein the specific surface area of the electrode active material is 10m²More than 25 m/g²(ii)/g or less, wherein the average particle diameter of spherical secondary particles obtained by granulating the primary particles of the electrode active material is 0.5 to 15 [ mu ] m, and the circularity measured by a flow-type particle image analyzer is 0.90 to 15 [ mu ] mThe content of the spherical secondary particles in the range of 0.95 or less is 18% or more by number ratio relative to the total number of particles including a single particle and all spherical secondary particles at the time of measuring circularity.

[2] The electrode material for a lithium ion secondary battery according to the above [1], wherein, among peaks appearing at 1 μm or less in a pore distribution diagram of the electrode active material measured using a mercury porosimeter, a peak having a largest pore volume has a peak top having a pore diameter of 0.03 μm or more and 0.14 μm or less.

[3] The electrode material for a lithium-ion secondary battery as recited in the above [1] or [2], wherein the olivine-structured transition metal lithium phosphate compound is an electrode active material represented by the following general formula (1).

Li_xA_yD_zPO₄(1)

(wherein A is at least 1 selected from the group consisting of Co, Mn, Ni, Fe, Cu and Cr, D is at least 1 selected from the group consisting of Mg, Ca, Sr, Ba, Ti, Zn, B, Al, Ga, In, Si, Ge, Sc and Y, 0.9 < x < 1.1, 0 < Y < 1, 0 < z < 1, 0.9 < Y + z < 1.1.)

[4]According to the above [1]To [3]]The electrode material for a lithium-ion secondary battery as claimed in any one of the above items, wherein the total pressure of 5t/250mm is applied to an aluminum current collector having a thickness of 30 μm, and the ratio of the total pressure to the total pressure of 90: 5: 5 a positive electrode mixture layer containing the electrode material, a conductive auxiliary agent and a binder, wherein the interfacial resistance value between the pressed positive electrode mixture layer and the aluminum current collector is 1 Ω · cm²Hereinafter, the volume resistance value of the positive electrode mixture layer after pressing is 5.0 Ω · cm or less.

[5]According to the above [4]]The electrode material for a lithium ion secondary battery, wherein the particle diameter (D90) of 90% cumulative volume percentage in the cumulative particle size distribution of the electrode material is 15 [ mu ] m or less, and the particle diameter is adjusted so that the total pressure of 5t/250mm is applied to an aluminum current collector having a thickness of 30 [ mu ] m at a weight ratio (electrode material: conductive auxiliary agent: binder): 90: 5: 5 when the positive electrode mixture layer containing the electrode material, the conductive auxiliary agent and the binder is pressed, the pressingThe ratio of the interfacial resistance value of the positive electrode mixture layer and the aluminum current collector after the production to the D90 (interfacial resistance value/D90) was 0.1. omega. cm²And a ratio of a volume resistance value of the positive electrode mixture layer after pressing to the D90 (volume resistance value/D90) is 0.10 Ω -cm/μm or more and 0.60 Ω -cm/μm or less.

[6] The electrode material for a lithium-ion secondary battery as recited in the above [4] or [5], wherein an oil absorption of the electrode material using N-methyl-2-pyrrolidone is 50ml/100g or less.

[7]According to the above [4]]To [6]]The electrode material for a lithium-ion secondary battery as claimed in any one of the above items, wherein the total pressure of 5t/250mm is applied to an aluminum current collector having a thickness of 30 μm, and the ratio of the total pressure to the total pressure of 90: 5: 5 when the positive electrode mixture layer containing the electrode material, the conductive auxiliary agent and the adhesive is pressed, the electrode density of the pressed positive electrode mixture layer is 1.4g/cm³The above.

[8] A lithium ion secondary battery comprising a positive electrode, a negative electrode and an electrolyte, wherein the positive electrode comprises a positive electrode mixture layer formed using the electrode material according to any one of the above-mentioned items [1] to [7], and the volume resistance value of the positive electrode mixture layer is 5.0 Ω & cm or less.

[9]According to the above [8]The lithium ion secondary battery, wherein the positive electrode mixture layer after pressing has an electrode density of 1.4g/cm³The above.

ADVANTAGEOUS EFFECTS OF INVENTION

According to the present invention, it is possible to provide an electrode material for a lithium ion secondary battery capable of reducing the volume resistance value of an electrode mixture layer and the interface resistance value of the electrode mixture layer and an aluminum current collector, and a lithium ion secondary battery having improved charge and discharge characteristics.

Detailed Description

Embodiments of the electrode material for a lithium ion secondary battery according to the present invention will be described.

The present embodiment is specifically described for better understanding of the gist of the present invention, and is not intended to limit the present invention unless otherwise specified.

< electrode Material for lithium ion Secondary Battery >

The electrode material for a lithium ion secondary battery according to the present embodiment is characterized by having an electrode active material containing an olivine-structured transition metal lithium phosphate compound and a carbonaceous coating film covering the electrode active material, and the electrode active material has a specific surface area of 10m²More than 25 m/g²And/g or less, wherein the spherical secondary particles obtained by granulating the primary particles of the electrode active material have an average particle diameter of 0.5 to 15 [ mu ] m, and the content of the spherical secondary particles having a circularity in the range of 0.9 to 0.95 as measured by a flow particle image analyzer is 18% or more in terms of number ratio to the total number of particles including a single particle and all the spherical secondary particles at the time of circularity measurement.

The electrode active material used in the present invention contains an olivine-structured transition metal lithium phosphate compound. The olivine-structured transition metal lithium phosphate compound is preferably an electrode active material represented by the following general formula (1) from the viewpoint of high discharge capacity and high energy density.

Li_xA_yD_zPO₄(1)

Among them, as A, Co, Mn, Ni and Fe are preferable, and Fe is more preferable.

As D, Mg, Ca, Sr, Ba, Ti, Zn, Al are preferable. When the electrode active material contains these elements, a positive electrode mixture layer that can realize a high discharge potential and high safety can be provided. Moreover, since the amount of resources is abundant, the selected material is preferable.

The specific surface area of the electrode active material was 10m²More than 25 m/g²A ratio of 10m or less per gram²More than g and 18m²A ratio of 10m or less per gram²A ratio of 1 to 1/g5m²The ratio of the carbon atoms to the carbon atoms is less than g. If the specific surface area is less than 10m²The Li ion diffusion resistance and the electron transfer resistance in the interior of the primary particles of the electrode material for a lithium ion secondary battery increase. Thus, the internal resistance increases, and the output characteristics deteriorate. On the other hand, if the specific surface area exceeds 25m²When the specific surface area of the electrode material for a lithium ion secondary battery is increased,/g, the mass of carbon required for increasing the specific surface area increases, and the battery capacity of the lithium ion secondary battery per unit mass of the electrode material for a lithium ion secondary battery is excessively decreased.

The specific surface area can be measured by a BET method using a specific surface area meter (e.g., product name: BELSORP-mini, manufactured by MicrotracBEL Corp.).

The average particle diameter of the primary particles of the electrode active material is preferably 500nm or less, more preferably 10nm or more and 400nm or less, still more preferably 20nm or more and 300nm or less, and still more preferably 20nm or more and 200nm or less. When the average particle diameter of the primary particles of the electrode active material is 500nm or less, it is possible to suppress an increase in lithium ion diffusion resistance or electron transfer resistance in the interior of the primary particles of the electrode active material. As a result, the lithium-ion secondary battery using the electrode material for a lithium-ion secondary battery of the present embodiment can increase the discharge capacity in high-rate charge and discharge. When the average particle diameter of the primary particles is 10nm or more, the increase in the required carbon mass can be suppressed by increasing the specific surface area of the primary particles of the electrode active material, and the decrease in the charge/discharge capacity per unit mass of the electrode material can be suppressed. Further, the surfaces of the primary particles of the electrode active material are easily and uniformly coated with the carbonaceous coating. As a result, the lithium ion secondary battery using the electrode material for a lithium ion secondary battery of the present embodiment has a large discharge capacity at high-rate charge and discharge, and can realize sufficient charge and discharge performance.

The average particle size is a volume average particle size. The average particle size of the primary particles can be calculated by randomly selecting 100 primary particles, measuring the particle size (major axis) of each primary particle with a Scanning Electron Microscope (SEM), and calculating the average value.

The average particle diameter of the spherical secondary particles obtained by granulating the primary particles of the electrode active material is preferably 0.5 μm or more and 15 μm or less, and more preferably 1.0 μm or more and 10 μm or less. When the average particle diameter of the spherical secondary particles is less than 0.5 μm, a large amount of the conductive aid and the binder is required when an electrode material, the conductive aid, a binder resin (binder) and a solvent are mixed to prepare an electrode material paste for a lithium ion secondary battery, and the battery capacity of the lithium ion secondary battery per unit mass of the positive electrode mixture layer for a lithium ion secondary battery is reduced. On the other hand, if the average particle diameter of the spherical secondary particles exceeds 15 μm, the dispersibility and uniformity of the conductive additive or binder in the positive electrode mixture layer are reduced. As a result, the lithium ion secondary battery using the electrode material for a lithium ion secondary battery of the present embodiment has insufficient discharge capacity at high-rate charge and discharge.

In the present embodiment, all of the granulated particles and the agglomerated particles except the primary particles existing alone (existing without agglomeration) are set to be secondary particles or spherical secondary particles.

The average particle size is a volume average particle size. The average particle diameter of the spherical secondary particles can be measured using a laser diffraction scattering particle size distribution measuring apparatus or the like. Alternatively, the average value of the major and minor diameters of each spherical secondary particle may be calculated by randomly selecting 100 spherical secondary particles, and measuring the major and minor diameters of each spherical secondary particle with a Scanning Electron Microscope (SEM). In the present invention, the particle diameter of the spherical secondary particles of the electrode active material coated with the carbonaceous coating (hereinafter also referred to as "carbonaceous-coated electrode active material") is calculated by the above method, and can be set to the average particle diameter of the spherical secondary particles of the electrode active material.

The content of the spherical secondary particles having a circularity in the range of 0.90 to 0.95 is 18% or more, preferably 19% or more, and more preferably 20% or more in terms of number ratio to the total number of particles including a single particle and all the spherical secondary particles at the time of circularity measurement. If the content is less than 18%, the electrode structure becomes uneven, and Li ion conductivity and electron conductivity may decrease.

Here, "circularity" is a value representing a shape close to a circle, and a maximum value of 1.0 represents a perfect circle. The circularity can be calculated by the following formula (I).

4πS²/L²(I)

Wherein, in the formula (I), S is the area of the spherical secondary particle, and L is the perimeter of the spherical secondary particle.

Among peaks appearing at 1 μm or less in a pore distribution diagram when an electrode active material is measured using a mercury porosimeter, the pore diameter of the peak top of the peak having the largest pore volume is preferably 0.03 μm or more and 0.14 μm or less, and more preferably 0.07 μm or more and 0.095 μm or less.

When the pore diameter of the peak top appearing at 1 μm or less is 0.03 μm or more and 0.14 μm or less, the penetration of the electrolyte solution is good, the ion resistance can be reduced, and the contact point between the active material particles is increased, and the electron conductivity in the micro region can be improved.

The carbonaceous coating film is used to impart desired electronic conductivity to the primary particles.

The thickness of the carbonaceous coating is preferably 0.5nm or more and 5.0nm or less, and more preferably 1.0nm or more and 3.0nm or less.

If the thickness of the carbonaceous coating is 0.5nm or more, the thickness of the carbonaceous coating is not excessively reduced, and a film having a desired resistance value can be formed. As a result, the conductivity is improved, and the conductivity as an electrode material can be secured. On the other hand, if the thickness of the carbonaceous coating is 5.0nm or less, the battery activity, for example, the battery capacity per unit mass of the electrode material can be suppressed from decreasing.

The amount of carbon contained in the carbonaceous coated electrode active material is preferably 0.5 mass% or more and 5.0 mass% or less, and more preferably 0.8 mass% or more and 2.5 mass% or less.

When the carbon content is 0.5 mass% or more, the conductivity as an electrode material can be secured, the discharge capacity at a high rate of charge and discharge when forming a lithium ion secondary battery becomes large, and sufficient charge and discharge rate performance can be achieved. On the other hand, if the carbon amount is 5.0 mass% or less, the carbon amount does not excessively increase, and it is possible to suppress an excessive decrease in the battery capacity of the lithium ion secondary battery per unit mass of the electrode material for a lithium ion secondary battery.

The coating rate of the carbonaceous coating with respect to the inorganic particles is preferably 60% or more, and more preferably 80% or more and 95% or less. The coating rate of the carbonaceous coating is 60% or more, and thus the coating effect of the carbonaceous coating can be sufficiently obtained.

The thickness of the carbonaceous coating can be measured using a transmission electron microscope.

The density of the carbonaceous coating, calculated from the amount of carbon in the carbonaceous coating, the average thickness of the carbonaceous coating, the coating rate of the carbonaceous coating, and the specific surface area of the electrode material, is preferably 0.3g/cm³Above and 1.5g/cm³Hereinafter, more preferably 0.4g/cm³Above and 1.0g/cm³The following.

The reason why the density of the carbonaceous coating is limited to the above range is that if the density of the carbonaceous coating calculated from the amount of carbon in the carbonaceous coating is 0.3g/cm³As described above, the carbonaceous coating film exhibits sufficient electronic conductivity. On the other hand, if the density of the carbonaceous coating film is 1.5g/cm³Since the amount of fine crystals of graphite having a layered structure in the carbonaceous coating is small, steric hindrance due to the fine crystals of graphite does not occur when lithium ions are diffused in the carbonaceous coating. This does not increase the lithium ion transfer resistance. As a result, the internal resistance of the lithium ion secondary battery does not increase, and a voltage drop does not occur at a high rate of charge and discharge of the lithium ion secondary battery.

The carbonaceous coated electrode active material is preferably a secondary particle whose particle shape is not easily deformed by electrode pressing, because the electrode structure can be made uniform. If the electrode structure is uniform, not only Li ion conductivity and electron conductivity are improved, but also the pressing pressure at the time of producing the electrode is suppressed, so that peeling of the carbonaceous coating due to collapse of the carbonaceous coated electrode active material can be suppressed, and the electrode mixture layer can be prevented from falling off from the aluminum current collector. This can suppress degradation of battery characteristics.

The particle diameter (D90) at which the cumulative volume percentage in the cumulative particle size distribution of the electrode material for a lithium ion secondary battery of the present embodiment is 90% is preferably 17 μm or less, more preferably 16 μm or less, and still more preferably 15 μm or less. When D90 is 17 μm or less, deformation of the secondary particles during electrode pressing can be suppressed. The lower limit of D90 is not particularly limited, but is preferably 4.0 μm or more, more preferably 6.0 μm or more, and still more preferably 7.0 μm or more. The cumulative particle size distribution refers to a volume-based cumulative particle size distribution. The cumulative particle size distribution of the electrode material can be measured using a laser diffraction scattering particle size distribution measuring apparatus or the like.

The oil absorption of N-methyl-2-pyrrolidone (NMP) using the electrode material is preferably 50ml/100g or less, more preferably 48ml/100g or less, and still more preferably 45ml/100g or less. When the NMP oil absorption is 50ml/100g or less, thickening of the electrode paste can be suppressed, and dispersion of the conductive auxiliary agent or the binder becomes easy. The lower limit of the oil absorption using NMP is not particularly limited, and is, for example, 20ml/100 g.

The NMP oil absorption can be measured by the method described in examples.

The electrode material for a lithium ion secondary battery of the present embodiment is formed on an aluminum current collector having a thickness of 30 μm, by applying a total pressure of 5t/250mm to a mixture of 90: 5: 5 the positive electrode mixture layer containing the electrode material, the conductive assistant and the binder is pressed, the interfacial resistance value between the pressed positive electrode mixture layer and the aluminum current collector can be preferably set to 1.50 Ω · cm²Hereinafter, more preferably 1.00. omega. cm²Hereinafter, it is more preferable to set the voltage to 0.90 Ω · cm²Hereinafter, it is more preferable to set the thickness to 0.80. omega. cm²It is more preferably set to 0.70. omega. cm²Hereinafter, the volume resistance value of the positive electrode mixture layer after pressing can be set to preferably 5.0 Ω · cm or less, more preferably 4.8 Ω · cm or less, and still more preferably 4.5 Ω · cm or less. In addition, the lower limit value of the interface resistance valueBut is not particularly limited, and is, for example, 0.005. omega. cm²。

The interface resistance value is a resistance value at an interface where two layers are in contact with each other, and in the present invention, is a resistance value at an interface between the positive electrode mixture layer and the aluminum current collector after pressing.

In the above conditions, the ratio of the interfacial resistance value between the positive electrode mixture layer and the aluminum current collector after pressing to D90 (interfacial resistance value/D90) can be preferably set to 0.1 Ω · cm²A value of 0.08. omega. cm or less, more preferably,/μm or less²A value of 0.05. omega. cm or less, more preferably,. mu.m or less²A value of less than μm. The ratio of the volume resistance value of the positive electrode mixture layer after pressing to D90 (volume resistance value/D90) can be preferably 0.10 Ω · cm/μm or more and 0.60 Ω · cm/μm or less, more preferably 0.11 Ω · cm/μm or more and 0.50 Ω · cm/μm or less, and still more preferably 0.12 Ω · cm/μm or more and 0.30 Ω · cm/μm or less. The lower limit of the interfacial resistance value/D90 is not particularly limited, but is, for example, 0.05. omega. cm²/μm。

The electrode material for a lithium ion secondary battery of the present embodiment is formed on an aluminum current collector having a thickness of 30 μm by applying a total pressure of 5t/250mm to a mixture of 90: 5: when the positive electrode mixture layer containing the electrode material, the conductive auxiliary agent, and the binder is pressed, the electrode density of the positive electrode mixture layer after pressing can be preferably set to 1.40g/cm³More preferably 1.50g/cm or more³The above. The upper limit of the electrode density is not particularly limited, but is, for example, 2.5g/cm³。

[ method for producing electrode Material ]

The method for producing an electrode material according to the present embodiment includes, for example: a step of producing an electrode active material and an electrode active material precursor; a slurry preparation step of mixing an electrode active material raw material selected from at least one of the group consisting of an electrode active material and an electrode active material precursor with water to prepare a slurry; a granulation step of adding a cohesion retention agent to the slurry obtained in the step to obtain a granulated substance; and a calcination step of dry-mixing the granulated substance obtained in the step with an organic compound of a carbonaceous coating precursor, and calcining the obtained mixture in a non-oxidizing environment.

(Process for producing electrode active Material and electrode active Material precursor)

The production process of the electrode active material and the electrode active material precursor is not particularly limited, and for example, when the electrode active material and the electrode active material precursor are represented by the general formula (1), conventional methods such as a solid phase method, a liquid phase method, and a gas phase method can be used. As Li obtained in this way_xA_yD_zPO₄For example, a particulate compound (hereinafter, sometimes referred to as "Li")_xA_yM_zPO₄Particles ". ).

Li_xA_yD_zPO₄The particles can be obtained, for example, by hydrothermal synthesis of a slurry-like mixture obtained by mixing a Li source, an a source, a P source, water, and, if necessary, a D source. According to hydrothermal synthesis, Li_xA_yD_zPO₄Formed as a precipitate in water. The precipitate obtained may be Li_xA_yD_zPO₄A precursor of (2). At this time, by calcining Li_xA_yD_zPO₄To obtain target Li_xA_yD_zPO₄Particles.

In the hydrothermal synthesis, a pressure-resistant sealed vessel is preferably used.

Here, the Li source may be lithium acetate (LiCH)₃COO), lithium salts such as lithium chloride (LiCl), and lithium hydroxide (LiOH). Among them, as the Li source, at least 1 selected from the group consisting of lithium acetate, lithium chloride and lithium hydroxide is preferably used.

Examples of the a source include chlorides, carboxylates, and sulfates containing at least 1 selected from the group consisting of Co, Mn, Ni, Fe, Cu, and Cr. For example, Li_xA_yD_zPO₄When A in (A) is Fe, the Fe source may be iron (II) chloride (FeCl)₂) Iron (II) acetate (Fe (CH)₃COO)₂) Iron (II) sulfate (FeSO)₄) And divalent iron salts. Among them, as the Fe source, at least 1 selected from the group consisting of iron (II) chloride, iron (II) acetate and iron (II) sulfate is preferably used.

Examples of the D source include chlorides, carboxylates, and sulfates containing at least one selected from the group consisting of Mg, Ca, Sr, Ba, Ti, Zn, B, Al, Ga, In, Si, Ge, Sc, and Y.

As the P source, phosphoric acid (H) may be mentioned₃PO₄) Ammonium dihydrogen phosphate (NH)₄H₂PO₄) Diammonium hydrogen phosphate ((NH)₄)₂HPO₄) And the like. Among them, as the P source, at least 1 selected from the group consisting of phosphoric acid, monoammonium phosphate and diammonium phosphate is preferably used.

(slurry preparation Process)

In this step, the electrode active material obtained in the above step is dispersed in water to prepare a uniform slurry. When the electrode active material raw material is dispersed in water, a dispersant may be added. The method of dispersing the electrode active material raw material in water is not particularly limited, and for example, a method using a medium stirring type dispersing apparatus for stirring medium particles at a high speed such as a planetary ball mill, a vibration ball mill, a bead mill, a paint stirrer, and an attritor is preferable.

When preparing the slurry, the ratio (D90/D10) of the particle diameter (D90) of 90% in cumulative volume percentage to the particle diameter (D10) of 10% in the cumulative particle size distribution of the electrode active material in the slurry may be controlled to be 1 to 30. When (D90/D10) is in the above range, the particle size distribution of the electrode active material in the slurry is broadened, and the density of the particles obtained can be increased, thereby exhibiting the effects of the present invention.

The dispersion conditions of the slurry can be adjusted by, for example, the concentration of the electrode active material in the slurry, the stirring speed, the stirring time, and the like.

(granulation Process)

In this step, a granulated substance is produced from the electrode active material in the slurry.

In granulation, collapse of the secondary particles is suppressed by causing the primary particles in the slurry to undergo soft aggregation, and a sample in which the content of spherical secondary particles having a circularity in the range of 0.90 to 0.95 is 18% by number or more relative to the total number of particles including single particles and all spherical secondary particles in the circularity measurement is easily obtained. As a method for promoting the soft aggregation of primary particles to suppress the collapse of the granules, in the present invention, an aggregation-maintaining agent is added to the slurry in the granulation step. Here, the aggregation maintaining agent is a compound that promotes aggregation of primary particles and maintains the shape of secondary particles formed by aggregation of the primary particles.

Examples of the method include a method in which an organic acid such as citric acid, polyacrylic acid, or ascorbic acid is added to the slurry as a coagulation-retaining agent and mixed. The pH of the slurry is lowered by the organic acid to promote the aggregation of the primary particles, so that secondary particles filled with more primary particles can be formed after granulation, and the strength of the secondary particles can be enhanced.

The reason why the organic acid is selected is because it is preferable that the organic acid is not left as an impurity in a firing step described later, but is carbonized as a part of the carbon coating layer. However, since the organic acid hardly retains carbon, it is difficult to form a carbon coating layer only by this method, and the addition of a large amount is not preferable in terms of difficulty in forming a good carbon coating layer.

In the calcination step described later, a carbonization catalyst for promoting carbonization of the organic compound may be used.

The amount of the coagulation maintaining agent is preferably 0.1 to 2.0% by mass, more preferably 0.2 to 1.5% by mass in terms of solid content, based on the electrode active material. By setting the content to 0.1 mass% or more, collapse of the granules can be suppressed. By setting the amount to 2.0 mass% or less, the increase in the amount of carbon derived from the aggregation-maintaining agent can be suppressed. If the amount of carbon is large, lithium ion conductivity may become insufficient.

The concentration of the electrode active material contained in the slurry is preferably 15 to 80 mass%, more preferably 20 to 70 mass%, and spherical secondary particles can be obtained.

Next, the mixture obtained above is sprayed in a high temperature environment where the atmospheric temperature is not less than the boiling point of the solvent, for example, in an atmosphere of 100 to 250 ℃, and dried.

Here, by appropriately adjusting conditions at the time of spraying, for example, the concentration of the electrode active material in the slurry, the spraying pressure, the spraying speed, and conditions at the time of drying after spraying, for example, the temperature rise rate, the maximum holding temperature, the holding time, and the like, a dried product in which the average particle diameter of the spherical secondary particles is within the above range is obtained.

The atmospheric temperature at the time of spraying/drying affects the evaporation rate of the solvent in the slurry, and thus the structure of the obtained dried product can be controlled.

For example, the closer the atmospheric temperature is to the boiling point of the solvent in the slurry, the longer the time required for drying of the sprayed droplets is, and thus the obtained dried matter is sufficiently shrunk in the time period required for the drying. Thus, the dried product obtained by spraying and drying at an atmospheric temperature around the boiling point of the solvent in the slurry is likely to have a solid structure.

Further, the higher the atmospheric temperature is, the shorter the drying time of the sprayed droplets becomes, the higher the temperature is, the higher the boiling point of the solvent in the slurry, and therefore the obtained dried product cannot be sufficiently shrunk. This makes it easy for the dried product to have a porous structure or a hollow structure.

(calcination Process)

In this step, the granulated substance obtained in the above step is calcined in a non-oxidizing atmosphere.

First, before firing, the granulated substance is dry-mixed with an organic compound that is a precursor of the carbonaceous coating film.

The organic compound is not particularly limited as long as it is a compound capable of forming a carbonaceous coating on the surface of the positive electrode active material, and examples thereof include polyvinyl alcohol (PVA), polyvinylpyrrolidone, cellulose, starch, gelatin, carboxymethyl cellulose, methyl cellulose, hydroxymethyl cellulose, hydroxyethyl cellulose, polystyrene sulfonic acid, polyacrylamide, polyvinyl acetate, glucose, fructose, galactose, mannose, maltose, sucrose, lactose, glycogen, pectin, alginic acid, glucomannan, chitin, hyaluronic acid, chondroitin, agarose, polyether, glycol, triol, and the like. However, substances contained in the organic acid used in the above-mentioned granulating step are excluded. These may be used alone or in combination of two or more.

The mixing ratio of the organic compound and the electrode active material raw material is preferably 0.5 parts by mass or more and 2.5 parts by mass or less based on the carbon weight obtained from the organic compound with respect to 100 parts by mass of the active material obtained from the electrode active material raw material. The actual amount of the carbon source to be added varies depending on the amount of carbonization by heating (the type of the carbon source or the carbonization condition), but is about 0.7 to 6 parts by weight.

Next, the mixture obtained by the dry mixing is calcined in a non-oxidizing environment at a temperature of preferably 650 ℃ or more and 1000 ℃ or less, more preferably 700 ℃ or more and 900 ℃ or less for 0.1 hour or more and 40 hours or less.

As the non-oxidizing atmosphere, nitrogen (N) is preferably included₂) And an inert gas atmosphere such as argon (Ar). When it is desired to further suppress the oxidation of the mixture, it is preferable to contain hydrogen (H) in the order of several vol%₂) And a reducing atmosphere of a reducing gas. Oxygen (O) may be introduced into the non-oxidizing atmosphere for the purpose of removing organic components evaporated in the non-oxidizing atmosphere during the calcination₂) And the like combustion-supporting gases or combustible gases.

Here, by setting the calcination temperature to 650 ℃ or higher, the decomposition and reaction of the organic compound contained in the mixture are easily and sufficiently performed, and the carbonization of the organic compound is easily and sufficiently performed. As a result, the formation of a decomposition product of the organic compound having a high resistance in the obtained particles can be easily prevented. On the other hand, by setting the firing temperature to 1000 ℃ or lower, lithium (Li) in the electrode active material raw material is less likely to evaporate, and the growth of electrode active material particles can be suppressed to a target size or larger. As a result, when a lithium ion secondary battery including a positive electrode including the electrode material of the present embodiment is manufactured, it is possible to prevent a decrease in discharge capacity at a high charge/discharge rate and realize a lithium ion secondary battery having sufficient charge/discharge rate performance.

[ lithium ion Secondary Battery ]

The lithium ion secondary battery of the present embodiment has a positive electrode, a negative electrode, and an electrolyte. The positive electrode is characterized by having a positive electrode mixture layer formed using the electrode material, and the positive electrode mixture layer has a volume resistance value of 5.0 Ω · cm or less.

If the volume resistance value of the positive electrode mixture layer is greater than 5.0 Ω · cm, the electron conductivity may decrease. The volume resistance value of the positive electrode mixture layer is preferably 4.8 Ω · cm or less, and more preferably 4.5 Ω · cm or less. The lower limit of the volume resistance value of the positive electrode mixture layer is not particularly limited, and is, for example, 0.5 Ω · cm.

The volume resistance value can be measured by the method described in the examples.

The interfacial resistance value between the positive electrode mixture layer and the aluminum current collector is preferably 1 Ω · cm²Hereinafter, more preferably 0.8. omega. cm²Hereinafter, more preferably 0.5. omega. cm²More preferably 0.1. omega. cm²The following. When the interfacial resistance value between the positive electrode mixture layer and the aluminum current collector is 1. omega. cm²The electron conductivity can be improved as follows. The lower limit of the interfacial resistance value between the positive electrode mixture layer and the aluminum current collector is not particularly limited, and is, for example, 0.005 Ω · cm²。

The interfacial resistance value can be measured by the method described in the examples.

The electrode density of the positive electrode mixture layer after pressing is preferably 1.4g/cm³Above, more preferably 1.5g/cm³The above. If the density of the electrode after pressing of the positive electrode mixture layer is 1.4g/cm³As described above, the electron conductivity can be improved. The upper limit of the electrode density is not particularly limited, but is, for example, 2.5g/cm³。

The electrode density can be measured by the method described in the examples.

[ Positive electrode ]

In the case of producing a positive electrode, the electrode material, a binder including a binder resin, and a solvent are mixed to prepare a positive electrode forming coating material or a positive electrode forming paste. In this case, a conductive aid such as carbon black, acetylene black, graphite, ketjen black, natural graphite, or artificial graphite may be added as necessary.

As the binder resin, for example, Polytetrafluoroethylene (PTFE) resin, polyvinylidene fluoride (PVdF) resin, fluororubber, or the like is preferably used.

The mixing ratio of the electrode material and the binder resin is not particularly limited, and for example, the binder resin is 1 to 30 parts by mass, preferably 3 to 20 parts by mass, based on 100 parts by mass of the electrode material.

The solvent used in the positive electrode forming coating material or the positive electrode forming paste may be appropriately selected by blending the properties of the binder resin.

For example, water can be lifted; alcohols such as methanol, ethanol, 1-propanol, 2-propanol (isopropyl alcohol: IPA), butanol, pentanol, hexanol, octanol, and diacetone alcohol; esters such as ethyl acetate, butyl acetate, ethyl lactate, propylene glycol monomethyl ether acetate, propylene glycol monoethyl ether acetate, and γ -butyrolactone; ethers such as diethyl ether, ethylene glycol monomethyl ether (methyl cellosolve), ethylene glycol monoethyl ether (ethyl cellosolve), ethylene glycol monobutyl ether (butyl cellosolve), diethylene glycol monomethyl ether, and diethylene glycol monoethyl ether; ketones such as acetone, Methyl Ethyl Ketone (MEK), methyl isobutyl ketone (MIBK), acetylacetone, and cyclohexanone; amides such as dimethylformamide, N-dimethylacetoacetamide and N-methyl-2-pyrrolidone; and glycols such as ethylene glycol, diethylene glycol, and propylene glycol. These solvents may be used alone or in combination of two or more.

Next, a positive electrode-forming coating material or a positive electrode-forming paste is applied to one surface of an aluminum foil, and then dried to obtain an aluminum foil having a coating film formed on one surface, the coating film including a mixture of the electrode material and a binder resin.

Next, the coating film was pressed and pressed under pressure and dried, thereby producing a current collector (electrode) having an electrode material layer on one surface of the aluminum foil.

Thus, a positive electrode capable of reducing the direct current resistance and increasing the capacitance can be manufactured.

[ negative electrode ]

Examples of the negative electrode include carbon materials containing metallic Li, natural graphite, hard carbon black, and the like, Li alloys, and Li₄Ti₅O₁₂、Si(Li_4.4Si), etc.

[ electrolyte ]

The electrolyte is not particularly limited, but is preferably a nonaqueous electrolyte, and examples thereof include the following: ethylene carbonate (ethylene carbonate; EC) and ethyl methyl carbonate (ethyl methyl carbonate; EMC) were mixed so that the volume ratio thereof reached 1: 1, and dissolving lithium hexafluorophosphate (LiPF) in the obtained mixed solvent₆) So that the concentration thereof becomes, for example, 1 mol/dm³。

[ diaphragm ]

The positive electrode of the present embodiment and the negative electrode of the present embodiment can face each other with a separator interposed therebetween. As the separator, for example, porous propylene can be used.

Also, instead of the nonaqueous electrolyte and the separator, a solid electrolyte may be used.

In the lithium ion secondary battery of the present embodiment, since the positive electrode has the positive electrode mixture layer formed using the electrode material for a lithium ion secondary battery of the present embodiment, the volume resistance value of the electrode mixture layer and the interface resistance value between the electrode mixture layer and the aluminum current collector can be reduced, and the charge/discharge characteristics can be improved.

Examples

The present invention will be specifically described below with reference to examples and comparative examples. The present invention is not limited to the embodiments described in the examples.

[ Synthesis of Positive electrode Material for lithium ion Secondary Battery ]

(example 1)

Lithium phosphate (Li) as Li source and P source₃PO₄) And Iron (II) sulfate (FeSO) as Fe source₄) Mixed to form a mixture of Li: fe: p is 3: 1: 1. also, for mixed preparationDistilled water, 600ml of raw material slurry was prepared.

Then, the raw material slurry was stored in a pressure-resistant sealed container, and after hydrothermal synthesis at 180 ℃ for 2 hours, it was cooled to room temperature (25 ℃) to obtain cake-like electrode active material particles precipitated in the container. After the electrode active material particles were sufficiently washed with distilled water several times, the electrode active material particles and distilled water were mixed so that the concentration of the electrode active material particles became 60 mass%, thereby preparing a suspension slurry.

This suspension slurry was charged into a sand mixer together with zirconia balls having a diameter of 0.1mm, and the treatment time of the sand mixer was adjusted so that the ratio (D90/D10) of the particle diameter at 90% cumulative volume (D90) to the particle diameter at 10% cumulative volume (D10) in the cumulative particle size distribution of the electrode active material particles in the suspension slurry became 2, and dispersion treatment was performed.

Next, the slurry subjected to the dispersion treatment was mixed with a citric acid aqueous solution previously adjusted to 30 mass% in terms of citric acid solid content relative to the electrode active material particles, and further with distilled water so that the concentration of the electrode active material particles in the slurry was 50 mass%, followed by spraying and drying in an air atmosphere at 180 ℃.

Then, a polyvinyl alcohol powder was dry-mixed with the obtained dried product in an amount of 3.5 mass% relative to the electrode active material particles, and heat treatment was performed at 750 ℃ for 1 hour under an inert atmosphere to load carbon on the electrode active material particles, thereby producing a positive electrode material for a lithium ion secondary battery of example 1.

(example 2)

A positive electrode material for a lithium-ion secondary battery of example 2 was produced in the same manner as in example 1, except that a citric acid aqueous solution previously adjusted to 30 mass% was mixed with the slurry subjected to the dispersion treatment by the sand mixer in an amount of 1.0 mass% in terms of citric acid solid content with respect to the electrode active material particles, and distilled water was further mixed so that the concentration of the electrode active material particles in the slurry was 25 mass%.

(example 3)

A positive electrode material for a lithium ion secondary battery of example 3 was produced in the same manner as in example 1, except that the suspension slurry adjusted to have an electrode active material particle concentration of 60 mass% was charged into a sand mixer together with zirconia balls having a diameter of 1mm, and the treatment time of the ball mill was adjusted so that the ratio (D90/D10) of the electrode active material particles in the suspension slurry became 25 to perform dispersion treatment.

(example 4)

A positive electrode material for a lithium-ion secondary battery of example 4 was produced in the same manner as in example 3, except that a citric acid aqueous solution adjusted in advance to be 30 mass% was mixed with the slurry subjected to the dispersion treatment by the sand mixer in an amount of 1.0 mass% in terms of citric acid solid content with respect to the electrode active material particles, and distilled water was further mixed so that the concentration of the electrode active material particles in the slurry was 25 mass%.

(example 5)

A slurry subjected to dispersion treatment by a sand mixer was mixed with a citric acid aqueous solution previously adjusted to 30 mass% in terms of citric acid solid content relative to the positive electrode active material particles, and further with distilled water so that the concentration of the positive electrode active material particles in the slurry was 50 mass%, and thereafter, the resultant was sprayed and dried in an air atmosphere at 180 ℃.

A positive electrode material for a lithium-ion secondary battery of example 5 was produced in the same manner as in example 1, except that glucose powder was dry-mixed in an amount of 4.7 mass% based on the electrode active material particles into the obtained dried product.

(example 6)

A positive electrode material for a lithium-ion secondary battery of example 6 was produced in the same manner as in example 5, except that a citric acid aqueous solution previously adjusted to 30 mass% was mixed with the slurry subjected to the dispersion treatment by the sand mixer in an amount of 1.0 mass% in terms of citric acid solid content with respect to the electrode active material particles, and distilled water was further mixed so that the concentration of the electrode active material particles in the slurry was 25 mass%.

Comparative example 1

A positive electrode material for a lithium ion secondary battery of comparative example 1 was produced in the same manner as in example 1, except that the slurry subjected to the dispersion treatment by the sand mixer was mixed with a polyvinyl alcohol aqueous solution previously adjusted to 15 mass% in terms of polyvinyl alcohol solid content with respect to the electrode active material particles, and distilled water was mixed so that the concentration of the electrode active material particles in the slurry was 50 mass%, and then the mixture was sprayed and dried in an environment of 180 ℃.

Comparative example 2

A positive electrode material for a lithium-ion secondary battery of comparative example 2 was produced in the same manner as in comparative example 1, except that a polyvinyl alcohol aqueous solution previously adjusted to 15 mass% was mixed with the slurry subjected to the dispersion treatment by the sand mixer in an amount of 3.5 mass% in terms of polyvinyl alcohol solid content with respect to the electrode active material particles, and distilled water was mixed so that the concentration of the electrode active material particles in the slurry was 25 mass%.

Comparative example 3

After washing cake-like electrode active material particles obtained by hydrothermal synthesis with distilled water sufficiently for many times, the electrode active material particles and distilled water were mixed so that the concentration of the electrode active material particles became 60 mass%, thereby preparing a suspension slurry. Next, a positive electrode material for a lithium ion secondary battery of comparative example 3 was produced in the same manner as in example 1, except that the suspension slurry was not subjected to dispersion treatment and a citric acid aqueous solution was not mixed.

Comparative example 4

After washing cake-like electrode active material particles obtained by hydrothermal synthesis with distilled water sufficiently for many times, the electrode active material particles and distilled water were mixed so that the concentration of the electrode active material particles became 60 mass%, thereby preparing a suspension slurry. Next, a positive electrode material for a lithium ion secondary battery of comparative example 4 was produced in the same manner as in example 6, except that the suspension slurry was not subjected to dispersion treatment and a citric acid aqueous solution was not mixed.

[ evaluation of Positive electrode Material ]

The obtained positive electrode material was evaluated by the following method. The results are shown in Table 1.

(1) A degree of circularity of 0.9 to 0.95

0.05g of the positive electrode active material particles were put into a 100ml plastic bottle, 100ml of distilled water and one spoon of a surfactant (CHARMY CLEAN manufactured by Lion Corporation) were put into the bottle, the bottle was covered with a lid, and after 10 times of shaking, 2ml of the solution was taken out by a glass pipette and put into a sample inlet of a flow type particle image analyzer (FPIA 3000S manufactured by Sysmex Corporation), and measurement was performed.

The measurement conditions were that the particle sheath was used as the sheath fluid, the measurement mode was HPF, the total particle count was 1000, and the standard lens was used as the lens.

(2) Pore distribution measurement

Mercury was injected into a cell into which 0.2g of positive electrode active material particles were charged in a low pressure mode using a mercury porosimeter (POREMASTER, manufactured by Quantachrome Instruments Japan g.k.) and the pore distribution was measured in a high pressure mode. The high pressure mode conditions were set to a lower pressure of 20PSI and an upper pressure of 60000 PSI.

(2) Average particle diameter of spherical secondary particles

The average particle diameter of the spherical secondary particles was calculated from the measurement values by observing the spherical secondary particles with a Scanning Electron Microscope (SEM), selecting 100 spherical secondary particles at random from the obtained SEM image, measuring the major axis and the minor axis of each particle, and taking (major axis + minor axis)/2 as the particle diameter.

(3) Particle size at 90% cumulative volume in cumulative particle size distribution (D90)

The measurement was performed using a laser diffraction particle size distribution measuring apparatus (product name: SALD-1000, manufactured by Shimadzu Corporation).

(4) Specific surface area

Using a specific surface area meter (product name: BELSORP-mini, manufactured by MicrotracBEl Corp.), byBased on nitrogen (N)₂) The specific surface area of the electrode material was measured by the adsorption BET method.

(5) Oil absorption by N-methyl-2-pyrrolidone (NMP) (NMP oil absorption)

The oil absorption using N-methyl-2-pyrrolidone (NMP) was measured by a method in accordance with JIS K5101-13-1 (method for purifying linseed oil) using NMP instead of purified linseed oil.

[ manufacture of Positive electrode ]

The mass ratio of the components is 90: 5: the obtained positive electrode material, polyvinylidene fluoride (PVdF) as a binder, and Acetylene Black (AB) as a conductive additive were mixed in the form of 5, and N-methyl-2-pyrrolidone (NMP) as a solvent was added thereto to impart fluidity, thereby preparing a slurry.

Subsequently, the slurry was coated on an aluminum (Al) foil having a thickness of 30 μm and dried. Thereafter, the positive electrodes of the respective examples and comparative examples were produced by applying a total pressure of 5t/250mm and pressing.

[ evaluation of Positive electrode ]

The obtained positive electrode was evaluated by the following method. The results are shown in Table 1.

(6) Density of electrode after pressing

Punching the positive electrode pressurized with the total pressure of 5t/250mm by a coin punch

The thickness of the positive electrode punched at 5 was measured, and the value obtained by subtracting the thickness of the current collector from the average value thereof was defined as the thickness of the positive electrode, thereby calculating the volume of the positive electrode. Similarly, the mass of the positive electrode was calculated from the difference in mass between the electrode and the current collector, and divided by the positive electrode volume to obtain the electrode density after pressing.

(7) Interfacial resistance value between positive electrode mixture layer and aluminum current collector

Using an electrode resistance measuring instrument (manufactured by HIOKI e.e. corporation, product name: XF057-012), at an applied current value: 1mA, voltage range: 0.2V, measurement speed: the assay was performed under Normal (standard) conditions. The voltage range is arbitrarily adjusted within a range in which the resistance value is not overloaded.

(8) Volume resistance value of positive electrode mixture layer

Using an electrode resistance measuring instrument (manufactured by HIOKI e.e. corporation, product name: XF057-012), after applying a printing current value: 1mA, voltage range: 0.2V, measurement speed: the assay was performed under Normal (standard) conditions. The voltage range is arbitrarily adjusted within a range in which the resistance value is not overloaded.

[ production of lithium ion Secondary Battery ]

A lithium metal was disposed as a negative electrode on the positive electrode of the lithium ion secondary battery obtained as described above, and a separator made of porous polypropylene was disposed between the positive electrode and the negative electrode to serve as a battery member.

On the other hand, the ratio of 1: 1 (mass ratio) ethylene carbonate and diethyl carbonate were mixed, and 1M LiPF was further added₆Thus, an electrolyte solution having lithium ion conductivity was prepared.

Next, the above-described battery member was immersed in the above-described electrolyte solution, thereby producing lithium ion secondary batteries of examples and comparative examples.

[ evaluation of lithium ion Secondary Battery ]

The obtained lithium ion secondary battery was evaluated by the following method. The results are shown in Table 1.

(9) Initial discharge capacity

The charge-discharge test of the lithium ion secondary battery was repeated 3 times at a constant current (10-hour charge and 10-hour discharge) of room temperature (25 ℃), a cut-off voltage of 2.5V to 3.7V, and a charge-discharge rate of 0.1C, and the third discharge capacity was set as the initial discharge capacity.

(10) Load characteristics (discharge capacity ratio)

After the initial discharge capacity was measured, the lithium ion secondary battery was charged at room temperature (25 ℃), a cut-off voltage of 2.5V to 3.7V, and 0.2C (5-hour charge), and then discharged at 3C (20 minutes discharge) to measure the discharge capacity.

The ratio of the discharge capacity at 3C to the discharge capacity at 0.1C (initial discharge capacity) was set as a load characteristic, and the load characteristic (discharge capacity ratio) was calculated by the following formula (1).

Discharge capacity ratio (%) (3C discharge capacity/0.1C discharge capacity) × 100 … … (1)

(11) DC Resistance (DCR; Direct Current Resistance)

The direct current resistance was measured using a lithium ion secondary battery in which the charging depth was adjusted to 50% (SOC 50%) at a constant current at a charging rate of 0.1C at an ambient temperature of 0 ℃. In a lithium ion secondary battery adjusted to SOC 50% at room temperature (25 ℃), currents at rates of 1C, 3C, 5C, and 10C were alternately applied to the charging side and the discharging side for 10 seconds, the current values after 10 seconds at the rates were plotted on the horizontal axis and the voltage values were plotted on the vertical axis, and the slopes of the approximate straight lines by the least squares method were defined as "charging side input DCR" and "discharging side output DCR". In addition, a 10-minute pause time was set for each of the change of the current-carrying direction and the change of the current-carrying current.

[ Table 1]

[ Table 2]

	Initial discharge capacity	Characteristic of load	Input DCR	Output DCR
						mAh/g	％	Ω	Ω
Example 1	138	96.8	3.1	2.7
					Example 2	140	97.1	2.6	2.6
Example 3	135	93.9	3.2	2.8
					Example 4	137	96.6	3.1	2.8
Example 5	138	97.4	2.8	2.7
					Example 6	140	96.8	2.6	2.5
Comparative example 1	126	80.3	6.2	3.4
					Comparative example 2	126	80.7	4.7	3.4
Comparative example 3	123	75.7	7.6	4.2
					Comparative example 4	125	77.2	6.2	4.5

(conclusion of the results)

As can be seen from the results in tables 1 and 2, when examples 1 to 6 and comparative examples 1 to 4 were compared, the positive electrodes for lithium ion secondary batteries of examples 1 to 6 had low interfacial resistance values between the positive electrode mixture layer and the aluminum current collector and low volume resistance values of the positive electrode mixture layer. Furthermore, it was confirmed that the lithium ion secondary batteries of examples 1 to 6 had a low DC resistance value, an initial discharge capacity,And excellent load characteristics. From this, it was found that an electrode material for a lithium ion secondary battery, which comprises an electrode active material comprising an olivine-structured transition metal lithium phosphate compound and a carbonaceous coating covering the electrode active material, was used, and the specific surface area of the electrode active material was 10m²More than 25 m/g²And/g or less, wherein the spherical secondary particles obtained by granulating the primary particles of the electrode active material have an average particle diameter of 0.5 to 15 [ mu ] m, and the content of the spherical secondary particles having a circularity in the range of 0.90 to 0.95 as measured by a flow particle image analyzer is 18% or more by number relative to the total number of particles including a single particle and all the spherical secondary particles at the time of circularity measurement, thereby obtaining a lithium ion secondary battery having a low interfacial resistance value between the positive electrode mixture layer and the aluminum current collector and a low volume resistance value of the positive electrode mixture layer. Further, it is also found that by using the electrode material for a lithium ion secondary battery, a lithium ion secondary battery having a low direct current resistance value and excellent initial discharge capacity and load characteristics can be obtained.

Claims

1. An electrode material for a lithium ion secondary battery, comprising:

an electrode active material comprising an olivine-structured transition metal lithium phosphate compound; and

a carbonaceous coating film covering the electrode active material,

the specific surface area of the electrode active material was 10m²More than 15 m/g²(ii) a ratio of the total of the components in terms of the ratio of the total of the components to the total of the components in the total,

wherein the average particle diameter of spherical secondary particles obtained by granulating the primary particles of the electrode active material is 0.5 to 15 [ mu ] m,

the content of spherical secondary particles having a circularity in the range of 0.90 to 0.95 measured by a flow particle image analyzer is 18% by number or more relative to the total number of particles including a single particle and all spherical secondary particles at the time of measuring the circularity,

among peaks appearing at 1 μm or less in a pore distribution diagram of the electrode active material measured by a mercury porosimeter, the peak having the largest pore volume has a pore diameter at the peak top of 0.03 μm or more and 0.14 μm or less,

wherein D90, which is a particle diameter at 90% cumulative volume percentage in the cumulative particle size distribution of the electrode material, is 15 [ mu ] m or less,

on an aluminum current collector having a thickness of 30 μm, a total pressure of 5t/250mm was applied to the electrode material: conductive auxiliary agent: the adhesive is 90: 5: 5, a positive electrode mix layer containing the electrode material, a conductive assistant and a binder, wherein the positive electrode mix layer has an interfacial resistance value of 1 Ω · cm and the aluminum current collector²The volume resistance value of the positive electrode mixture layer after pressing is 5.0 Ω · cm or less, and the interfacial resistance value/D90, which is the ratio of the interfacial resistance value between the positive electrode mixture layer after pressing and the aluminum current collector to D90, is 0.1 Ω · cm or less²And a volume resistance value/D90, which is a ratio of the volume resistance value of the positive electrode mixture layer after pressing to the D90, is 0.10 Ω -cm/μm or more and 0.60 Ω -cm/μm or less.

2. The electrode material for a lithium-ion secondary battery according to claim 1,

the olivine-structured transition metal lithium phosphate compound is an electrode active material represented by the following general formula (1),

Li_xA_yD_zPO₄(1)

wherein A is at least 1 selected from the group consisting of Co, Mn, Ni, Fe, Cu and Cr, D is at least 1 selected from the group consisting of Mg, Ca, Sr, Ba, Ti, Zn, B, Al, Ga, In, Si, Ge, Sc and Y, x is greater than 0.9 and less than 1.1, Y is greater than 0 and less than 1, z is greater than or equal to 0 and less than 1, and Y + z is greater than 0.9 and less than 1.1.

3. The electrode material for a lithium-ion secondary battery according to claim 1,

the oil absorption of the electrode material using N-methyl-2-pyrrolidone is 50ml/100g or less.

4. The electrode material for a lithium-ion secondary battery according to claim 1,

on an aluminum current collector having a thickness of 30 μm, a total pressure of 5t/250mm was applied to the electrode material: conductive auxiliary agent: the adhesive is 90: 5: 5, the positive electrode mixture layer containing the electrode material, the conductive assistant and the binder has an electrode density of 1.4g/cm³The above.